This invention is concerned with a method for optically measuring a distance about three dimentioned objects, and more particularly with a precise and continuous thickness measuring method of steel strip using a light beam.
Conventional methods of this kind have very precise accuracy such as method using light interference, a method using holography, a method using more image interference fringes and others. But these methods are available for laboratory use but not suitable for on-line use in a plant.
In these optical geometry measuring methods, the optical measuring method of triangulation is most suitable for on-line use. The measuring theory of the triangulation will first be described for better understanding of the invention referring to FIG. 1, in which a laser beam A is projected on a base line 1 toward a mirror 2 from which the reflected beam B is irradiated on an object 3 making a spot P on the object. Spot P is detected by a photo-detector 4 by way of a detecting optical axis 7 defined by a pair of slits 5 and 6. The distance M between spot P and base line 1 can be calculated with the distance l between the intersecting point Q.sub.1 of detecting optical axis 7 to base line 1 and the reflecting point Q.sub.2 of laser beam A at mirror 2, the intersecting angle .phi..sub.1 of detecting optical axis 7 to base line 1, and the intersecting angle .phi..sub.2 of reflecting light line 6 to laser beam A, if they are known. In a practical method, mirror 2 is fixed so that angle .phi..sub.2 has a constant value and, moving the set of photo-detector 4 and slits 5 and 6 parallel to base line 1, keeping angle .phi..sub.1 constant, distance l is determined with the displacement of the set until photo-detector 4 detects spot P, whereby the distance M is calculated. In another practical method, the set of photo-detector 4 and slits 5 and 6 is fixed to keep angle .phi..sub.1 constant and mirror 2 rotates to sweep the reflected light beam B on object 3. In this method, the distance M can be calculated using angle .phi..sub.1, angle .phi..sub.2 which is equal to 2.phi..sub.3 (.phi..sub.3 is the rotating angle of mirror 2 referred to base line 1 when spot P is detected by photo-detector 4), and distance l = d.sub.2 - d.sub.1 .times. sec .phi..sub.3 - d.sub.3 tan .phi..sub.3, d.sub.1 indicating the distance between the rotating axis of mirror 2 and the mirror surface, d.sub.2 indicating the distance between the rotating axis and intersecting point Q.sub.1 and d.sub.3 indicating the distance between the rotating axis and base line 1.
In this latter method, high accurate measurement of e.g., 2.mu.m unit requires very stable values in distances d.sub.1, d.sub.2 and d.sub.3 and in angle .phi..sub.1 with high precision measurement of rotating angle .phi..sub.3 of the mirror. Distance d.sub.1 is rather stable but d.sub.2, d.sub.3 and .phi..sub.1 are strictly unstable due to their thermal displacements. Particularly in measurement of profile of the object continuously moving the mirror and the photo detector along the base line, it becomes extremely difficult to keep d.sub.3 and .phi..sub.1 just constant. .phi..sub.1 and d.sub.3 should in fact be treated as variables. There is a similar problem in the former conventional method, in which distance l and angle .phi..sub.1 are also variables due to error in the photo-detector movement.